LOCK by block, street by bloody street, the good people of New York fear they’re losing their newly won grasp on this city.

The attacks at first seem isolated, random.

In January, Kendra Webdale is pushed to her death beneath a subway train.

In June, an elderly Guo-Xi Li is stabbed to death by a deranged woman on a Brooklyn street.

Soon, the crimes seem to come at an accelerated pace.

Police Officer Gerald Carter is shot to death in Staten Island by a teen-ager free on parole. And walking through Midtown in the afternoon daylight, young Nicole Barrett has her skull bashed in by a brick-wielding vagrant.

Not long ago, this city was ready to declare an end to crime.

Now, we are experiencing a real sense that the streets of New York are once again becoming the province of criminals and crazies.

“I left New York for 20 years, in part because I was afraid,” said Casey Burton, a Brooklyn mother of two.

“Now, I’m starting to feel very vulnerable again. I can’t always look behind my back while walking on the street. You can’t even walk in a crowd and feel comfortable — you can’t expect the crowd to save you.”

With each new violent revelation, we feel more helpless. With new each atrocity, criminals and lunatics feel bolder. And the cops, our only protectors, risk being cast as demons for doing their jobs.

In the end, it is the people of New York who are losing the turf fight.

Or maybe — thanks to politicians and activists who profit from anarchy — we have lost the will to win the war.

The perception that’s taking hold today is that the city is at a crossroads.

For the first time in six years, New York is posting a significant increase in murder. Through October, there were 592 homicides reported in the five boroughs, up from 521 last year.

How we deal with this uptick will determine our future as a city. In the last few years, word that our streets had grown virtually crime-free helped fuel a remarkable economic revival, with companies clamoring for office space, developers revitalizing desolate neighborhoods — and tourists unafraid to tread city pavement.

Today, we are a city tottering on the edge. It is not unimaginable to see New York slipping into the abyss of the Dinkins era — when annual murders peaked at 2,200 in 1991.

Since Mayor Giuliani took office, a very vocal cadre of politicians and activists, and the newspapers that love them, have argued that safer streets and less-scary subways are not something to enjoy.

Instead, they see the drop in crime as a kind of personal threat.

For a while, it was possible to ignore the complaints as the ravings of political losers in search of a job. But that changed with a trio of tragedies: the wrongful shooting by cops of Amadou Diallo, an unarmed African immigrant; the torture of Abner Louima by Officer Justin Volpe; and the fatal shooting of a deranged, hammer-wielding Gidone Busch.

It didn’t take long for these incidents to embolden political losers like Al Sharpton to indict all police by association. This summer, the parade of anti-cop protesters included wealthy, bodyguard-employing actors such as Susan Sarandon, plus the likes of Dinkins and state Comptroller Carl McCall.

A few months back, a Brooklyn police sergeant told me that, in the wake of Diallo, he and his colleagues have become so intimidated by the blanket outrage leveled against them, they sometimes walk away from tense situations.

Police play it safe. And the result is more dead bodies on the street.

In the meantime, activists, such as the American Civil Liberties Union, are working to empty the asylums. When a schizophrenic like Andrew Goldstein kills Kendra Webdale, his boosters insist he’s not responsible. But, by the same token, when cops shoot and kill a Gidone Busch, his ACLU friends rail that the cops should be indicted.

We can’t afford to turn over the streets to the predators.

Once you’ve enjoyed the sweet freedom of safety, there is no going back.